The present invention relates to process for increasing the quantity of powder dispensed per unit time to a powder coating system, wherein the powder is fed via a feed conduit from a feed point to a mixing chamber by producing along the feed conduit, by accelerating a gas jet in the mixing chamber, a pressure gradient oriented against the chamber, and by obtaining pressure recovery by retarding the powder-gas stream in order to feed the powder-gas stream through a conveying line to a coating arrangement, as well as to powder coating systems with a powder tank connected via a conduit with a mixing chamber, a conveying gas nozzle terminating in the latter in order to generate in the mixing chamber a vacuum with respect to the tank by gas jet acceleration, a conveying line for gas-powder mixture leading to a coating arrangement from this mixing chamber.
For coating substrates with a synthetic resin, two methods can be utilized, in principle. In the first method, a synthetic resin is present in a dissolved state. In this procedure, the solution is applied to the surface to be coated and the solvent is subsequently evaporated. In a second method, extremely fine synthetic resin powder is applied to a surface. In most cases, the application is enhanced by static electricity. The applied powder is then made to melt by the action of heat. Thereby, a synthetic resin layer that adheres to the surface is produced.
The present invention is basically concerned with the second-mentioned technique. In the latter, the difficulty encountered time and again resides in applying the synthetic resin powder uniformly to the surface to be coated, and for this purpose primarily a uniform conveyance of the powder is necessary. The powder cannot be transported over relatively long distances under the influence of gravity. A conveying medium, customarily a conveying gas, is required to be able to convey the powder along horizontal, or at least not only vertical distances. Specifically, the present invention thus relates to the aforementioned coating technique wherein relatively long conveying paths must be traversed between a powder tank and a coating unit.
Such a conventional coating system is illustrated schematically in FIG. 1. As indicated by arrow F, coating powder is filled into a storage tank 1. A feed conduit 3 connects the tank 1 with a mixing chamber 5. By means of a nozzle 7 terminating in the mixing chamber 5, a gas jet G produced by a pressure source 9, such as a compressor, is accelerated and injected into the mixing chamber 5.
The symbol p.sub.1 denotes the pressure in tank 1, p.sub.7 denotes the pressure in the feed conduit for the conveying gas G, and p.sub.5 is the pressure in the mixing chamber 5. Based on the jet acceleration at the nozzle 7, a pressure gradient .DELTA.p.sub.75 is produced. In the mixing chamber 5, a vacuum prevails initially with respect to the static pressure in the gas feed conduit 7. Due to the accelerated gas in the chamber 5, a static pressure gradient .DELTA.p.sub.15 is produced from tank 1 to chamber 5. Thereby, powder is transported from tank 1 into mixing chamber 5 and blended in mixing chamber 5 with the gas jet G. The powder-gas mixture is then discharged from the mixing chamber 5 through a conveying line 11. In a section 13 between mixing chamber 5 and conveying line 11, the powder-gas stream is decelerated whereby the kinetic energy in the mixing chamber 5 is converted into pressure energy and thus static pressure recovery occurs. By way of the conveying line 11, the powder-gas stream is then fed to a coating unit which is frequently relatively far remote from the tank 1, as is the case with the coating unit 21 illustrated schematically in the bottom portion of FIG. 1. This is a coating unit for the internal coating of pipes or structures having the shape of a tubular section, as utilized for the internal coating of can bodies, for example. In this process, first of all can bodies 15 not as yet joined along the longitudinal edges, i.e. not as yet closed all around, are fed by way of an overhanging arm first to a joining station 19, such as a welding station, and then further transported by way of arm 17 to a coating unit 21. The conveying line 11 for the coating powder is extended over the entire length of arm 17 axially through the latter and exits from the arm 17 only in the coating zone. At that location, enhanced by static electricity (not shown), the powder is applied to the inner surface of the can bodies 15 and excess powder is customarily removed again by suction for recovery. In this arrangement, it can be seen that relatively long conveying lines must be traveled between the so-called injector, generally denoted by 8, to the powder dispensing point.
On account of friction losses, there occurs a constant pressure drop in the powder-gas stream along the conveying line 11. These losses are represented in FIG. 1 by .DELTA.p.sub.11. These losses are overcome, inter alia, by the energy of the gas jet injected into the mixing chamber 5. This energy is sufficient for deflection and acceleration of the powder-gas mixture and for overcoming the aforementioned losses at the conveying line 11. It is an object of the present invention to increase the amount of powder dispensed per unit time to a coating unit, such as the arrangement 21 of FIG. 1. For this purpose, several measures present themselves, initially. Increasing the kinetic energy of the gas jet injected into the mixing chamber 5 which could be attained by raising the conveying pressure p.sub.7 or increasing the acceleration with the utilization of a Laval nozzle. The higher conveying pressure p.sub.7 has only a minor effect on the quantity of powder conveyed as soon as the pressure ratio at the nozzle, p.sub.7 /p.sub.5, becomes higher than the critical pressure ratio, which is about 1.7 in case air is used as the gas. Furthermore, increasing the conveying pressure p.sub.7 is extraordinarily expensive, since this pressure must in any event lie considerably above the pressure of the mixing chamber, p.sub.5, and the pressure p.sub.1 in the tank 1, which latter pressure is ordinarily atmospheric pressure.
The higher kinetic energy attained with the use of a Laval nozzle cannot be utilized on account of the poor degree of efficiency of jet expansion and, respectively, retardation and, respectively, on account of poor recovery of static pressure. An increase in the pressure ratio p.sub.7 /p.sub.5 above the critical proportion, such as by means of a Laval nozzle, furthermore results in unstable flow in the conveying line 11 due to the then occurring compression shock wave.
A further possibility for increasing the amount of powder dispensed would be enlarging the quantity of gas injected per unit time into the mixing chamber 5. With a given conveying line configuration, this leads to damming up and, respectively, pressure buildup in the mixing chamber 5 and to a pressure drop .DELTA.p.sub.11 over proportionate to the amount of powder conveyed through line 11.
The above-mentioned problem is solved, according to this invention, by partially compensating for a pressure drop in the powder-gas stream along the conveying line by raising the pressure at the feed point. Thereby, the pressure drop .DELTA.p.sub.11 according to FIG. 1 is not overcome by the energy of the gas jet injected through nozzle 7 but rather by the excess pressure p.sub.1 generated according to this invention in tank 1. The injected gas jet G now serves predominantly for the acceleration and guidance of the powder within the mixing chamber 5, and as a consequence the momentum of the gas jet injected through nozzle 7 can additionally be reduced substantially, with the amount dispensed being the same or being increased. By variation of the excess pressure p.sub.1 in tank 1, the conveyed quantity can be set.
Thus, a powder coating system of the invention of the type discussed hereinabove for solving the aforementioned problem, has a powder tank in communication with a mixing chamber via a conduit, a conveying gas nozzle terminating in the mixing chamber, in order to produce a vacuum with respect to the powder tank by gas jet acceleration in the mixing chamber, a conveying line for gas-powder mixture leading from the mixing chamber to a coating unit, characterized in that the powder tank is connected to a pressure source.
Furthermore, the amount of powder dispensed can herein be improved, with the use of a minimum quantity of gas passed through the nozzle, by optimizing the degree of jet expansion efficiency or, respectively, the degree of effective retardation. In other words, by optimizing the recovery of the static pressure at the aforementioned gas jet.
This has been attained in the process of the type described above by constantly decelerating the powder/gas stream in the mixing chamber and by mixing the powder through the feed conduit in the region of highest gas flow velocity with the gas stream.
As contrasted to the illustration according to FIG. 1 showing the structure, in principle, of conventional mixing chambers, the deceleration section is thus located in accordance with this invention directly adjacent to the nozzle orifice, which makes it possible to achieve the desired improvement in pressure recovery.
By way of explanation customarily, one speaks of jet expansion and/or compression in case of deceleration and/or acceleration of the jet. However, since, for example, in case of the Laval nozzle, the jet in the expansion zone or spreading zone is further accelerated, the behavior of the jet is herein described using the more unequivocal kinetic terms of "acceleration", "deceleration".
A powder coating system of the invention of the aforementioned type, for solving the problem stated above, has a powder tank in communication with a mixing chamber via a conduit, a conveying gas nozzle terminating in the mixing chamber, in order to produce a vacuum with respect to the tank by gas jet acceleration in the mixing chamber, a conveying line for gas-powder mixture leading from the mixing chamber to a coating unit characterized in that the mixing chamber has a section coaxial with respect to the axis of the nozzle and widening with constancy with respect to the nozzle orifice to the diameter of the conveying line.
As likewise shown in FIG. 1, it is conventional to extend the feed conduit 3 for the powder so that it is perpendicular, at least in one component, to the axis of the injected gas jet G of the mixing chamber 5. In this connection, there is a certain danger that powder settles in the mixing chamber, as 5 in FIG. 1. This leads to changes in the pressure and flow characteristics within the mixing chamber 5, and this, in turn, results in impairment of the dispensed powder quantity in the aforementioned sense. In order to solve this problem, which also has a negative effect on the above-mentioned powder quantity, it is further suggested according to the present invention that in a process of the type discussed above wherein a powder stream is fed from the feed conduit at least in one component perpendicularly to the axis of the gas jet of the mixing chamber, to introduce the powder stream eccentrically with respect to the axis of the gas jet into the mixing chamber to bring about a self-cleaning rotational flow of the powder-air stream. More specifically, according to the present invention, if in FIG. 1, the axis of the feed conduit 3 is extended so that, as seen in the direction toward the nozzle 7, the axis of the latter and the axis of conduit 3 do not intersect, then the flow of the powder-gas mixture becomes rotational. That is, a self-cleaning vortex is created in the mixing chamber 5 or, in any event, in the subsequent conduit 11.
A powder coating system of the invention of the aforementioned type which solves this last-discussed problem for obtaining an increased amount of powder dispensed, or of conveyed quantity, comprises a powder tank in communication with a mixing chamber via conduit, a conveying gas nozzle terminating in the mixing chamber, in order to produce a vacuum with respect to the tank by gas jet acceleration in the mixing chamber, a conveying line for the gas-powder mixture leading from the mixing chamber to a coating unit, wherein the conduit terminates in the mixing chamber with an axial direction being at least in one component perpendicular to the axial direction of the nozzle characterized in that the conduit terminates eccentrically with respect to the nozzle axis.
It is readily understood that an optimum effect in light of the problem on which this invention is based is attained by combining two or three of the aforementioned specific methods according to this invention and/or by combining two or more of the features of this invention inherent in the above described several powder coating systems of the invention.
If, according to the first-mentioned process of this invention, or according to a combination with this process, the pressure at the feed point into the feed conduit is raised, then it is further suggested to fluidize the powder upstream of the mixing chamber. This can take place in the tank, such as tank 1 according to FIG. 1 and/or along the feed conduit 3 between the tank and the mixing chamber and/or injector 8.
In a process wherein the powder-gas stream is constantly decelerated, as described above, it is furthermore proposed to continuously accelerate the gas jet by means of a nozzle, and to operate the nozzle with a subcritical pressure ratio.
By constant and, respectively, continuous acceleration, an axially parallel efflux is obtained at an exit cross section of the nozzle, such as the nozzle of FIG. 1.
By operating the nozzle with a pressure ratio smaller than the critical one, when using air as the gas at a ratio of p.sub.7 /p.sub.5 &lt;about 1.7, shock waves in the mixing chamber are avoided and the formation of a free jet is made possible. Furthermore, it is suggested for as optimal a pressure recovery as possible to allow the nozzle exit jet to decelerate to at least almost a free jet.
For ensuring a continuous, accumulation-free discharging of the powder through the feed conduit, such as conduit 3 in FIG. 1, it is suggested in the above-discussed powder coating system wherein the tank is connected to a pressure source, furthermore to provide a fluidizing plate in the zone where the conduit leaves the tank, and a feed line for a fluidizing gas. With preference, a conveying unit for the fluidizing gas is utilized also as the pressure source for raising the pressure in the tank.
Upon exposure of the aforementioned tank to excess pressure the problem arises that the tank must be charged time and again, or continuously, with powder without bringing about a pressure equalization between the tank and its surroundings.
This problem is solved by providing a powder charging conduit at the container, with a pressure decoupling arrangement, such as a cellular wheel sluice, for feeding powder from a pressure level on the inlet side to a pressure level on the tank side.
If furthermore, as mentioned, fluidizing air is introduced into the tank as a preferred feature for fluidizing the powder in the zone of the feed conduit, then measures must be taken making it possible to exhaust the thus-introduced air after the desired excess pressure has been attained. This is achieved by providing at the tank a pressure regulating arrangement, such as a pressure regulating valve, preferably with a filtering unit for suspended powder. Thereby, after establishing the desired excess pressure, the additionally introduced fluidizing air is removed and the powder particles that are in all cases suspended therein are filtered out by the filtering unit.
In a powder coating system according to this invention wherein the mixing chamber exhibits a section coaxial to the axis of the nozzle and constantly widening with respect to the nozzle orifice up to the cross section of the conveying line, it is further proposed to provide that the section flares at least almost to the extent of the jet boundary angle of a gas free jet forming at the nozzle, preferably with an angle of about 15.degree. or less, with respect to the nozzle axis. This permits optimum pressure recovery.
It is furthermore suggested in this system, in order to achieve optimum injection conditions at the mixing chamber with a minimum amount of gas and with a minimum gas conveying pressure, especially to realize an axially parallel flow in the region of the nozzle orifice, to provide a bore of the conveying gas nozzle which steadily narrows toward its orifice, the ratio of the diameters from the unconstricted section to the nozzle orifice being preferably larger than 5.
In order to be able furthermore to optimally adjust the conditions at the mixing chamber in dependence on the powder to be conveyed or in accordance with manufacturing tolerances, it is preferred that the nozzle in the mixing chamber be axially adjustable.
To provide for optimum influx and intermixing of the powder and of the gas jet with low losses and without interfering turbulences, it is furthermore provided that the conduit terminates in the chamber with a transverse component with respect to the nozzle axis and that a flow duct section of the chamber provides a continuous transition from the conduit entrance point into the deceleration section.
Preferably, this flow duct section and the subsequently flaring deceleration section constitute, similarly to a Laval nozzle of constant curvature, a constriction in the section facing the conveying line, the nozzle orifice lying in the zone of this constriction. Thereby, an annular nozzle is formed about the nozzle orifice projecting into the constriction zone, for introduction of the powder. Consequently, the powder is uniformly introduced along the mixing chamber periphery.